German Patent Application No. DE 101 39 142 describes an exhaust gas treatment apparatus of an internal combustion engine in which, in order to reduce NOx emissions, a selective catalytic reduction (SCR) converter is used which, with ammonia as the reducing agent, reduces the nitrogen oxides contained in the exhaust gas to nitrogen. The ammonia is obtained, in a hydrolysis catalytic converter disposed upstream from the SCR converter, from a urea-water solution. The hydrolysis catalytic converter reacts the urea contained in the urea-water solution with water to produce ammonia and carbon dioxide. To ensure precise dosing, provision is made for ascertaining the concentration of the urea-water solution.
The urea-water solution is brought to a predetermined pressure using a pump. A metering valve defines a predetermined flow rate. Compressed air is mixed into the urea-water solution in a mixing chamber. The urea-water solution is sprayed, together with the mixed-in air, into the exhaust gas of the internal combustion engine in such a way that a largely uniform inflow into the SCR converter is achieved.
A previously unpublished patent application of the Applicant describes a method and an apparatus of the species in which a pressurized reagent medium is likewise sprayed into the exhaust gas of an internal combustion engine upstream from an SCR converter. The reagent medium pressure is defined to a predetermined reagent medium target pressure as a function of a parameter. An operating variable of the internal combustion engine and/or a parameter of the exhaust gas of the internal combustion engine can be employed as a parameter. The predetermined reagent medium pressure target value is regulated in the context of a closed-loop control system in which the actual reagent medium pressure is sensed by a reagent medium pressure sensor. Compressed air can be mixed into the reagent medium. The compressed air pressure can likewise be regulated, as a function of the parameter and in the context of a closed-loop control system, to a predetermined compressed air pressure target value, the actual compressed air pressure being sensed by a compressed air pressure sensor. A defect in at least one of the pressure sensors can result in decreased SCR converter performance, with the consequence that unpurified exhaust gas can enter the environment.
It is an object of the present invention to provide a method for introducing a reagent medium into an exhaust gas conduit of an internal combustion engine, and an apparatus for carrying out the method, which ensure that at least one predetermined pressure is maintained.